Stirring of liquid steel in a continuous casting mold by means of an externally applied low-frequency A.C. electromagnetic field is a well-established practice in continuous casting steel operations. Electromagnetic stirring, commonly known as EMS, is broadly employed in continuous casting of steel in order to improve quality of the as cast product and process productivity. It was established that control of stirring motion in the region closely adjacent to the melt free surface, commonly termed meniscus, is required to meet the conditions of different casting practices. Thus a certain intensity stirring motion in the meniscus region is necessary in order to control surface and subsurface porosity, subsurface inclusions and other defects in billets and blooms produced from mainly Si—Mn deoxidized steel. The other casting practice, known as the submerged pouring under the mold powder, requires meniscus stability, and therefore it imposes restrictions to stirring motion at the meniscus. Besides these two opposing requirements stemmed from casting practices, the direct correlation was established between certain groups of strand defects and stirring intensity in both the meniscus region and the mold bulk. Therefore the need for control of stirring motion in those regions of the casting mold has necessitated a number of techniques based on application of D.C. or A.C. electromagnetic fields. Thus control of stirring motion in the meniscus region of the mold by means of a horizontal D.C. magnetic field was described in the U.S. Pat. No. 4,933,005. According to this patent, an externally applied D.C. magnetic field interacts with the stirring flow in the meniscus region originated by the main stirrer. An electromagnetic force results from this interaction and this force opposes liquid metal motion, thereby reducing the velocity of that motion. This method of stirring velocity control has limitations. Within the configuration common for the equipment used for continuous casting of billets and blooms, dimensions of the braking coil is restricted, as a result strength of D.C. magnetic field produced by the brake is sufficient only for reduction of stirring velocity at the meniscus up to 50 to 60 percent of the velocity original value.
Another known method in, prior art with the objective to control stirring motion in the meniscus region is a dual-coil EMS system operating with A.C. current and described in the U.S. Pat. No. 5,699,850. According to this patent, an induction coil arranged in the upper part of the mold in the meniscus region is energized from a current source independent from the current source of the main stirrer arranged in a lower portion of the mold. Thus a rotating A.C. magnetic field produced by the upper induction coil is independently controlled with respect to the magnetic field of the main stirrer. When the rotational direction of magnetic fields produced by the upper and the main stirrers coincides, stirring velocity in the meniscus region increases. This velocity increase can be controlled by the current input to the upper coil. In the case of rotational directions opposing each other, the upper stirrer becomes a magnetic brake with respect to the stirring flow in the meniscus region. By adjusting current of the brake, stirring velocity in the meniscus region can be controlled within a range from its original value when there is no braking action applied, to virtual zero, when magnetic torque of the brake is in balance with the angular momentum of stirring flow in the meniscus region.
As a downside of this method, the braking action has an effect only on the azimuthal component of the fluid flows induced by stirring or by the impact of pouring stream discharging into the mold. The longitudinal component of these flows remains unaffected by the A.C. magnetic field produced by the upper induction coil. These longitudinal fluid flows, depending on their intensity, produce a significant turbulence to the melt at the meniscus and in the region adjacent to the meniscus, therefore affecting operating conditions of casting practice and product quality.